Field of the Invention
The present invention relates to a coating device for manufacturing an optical fiber by applying a resin as a protective coating layer on a bare optical fiber and a method of manufacturing an optical fiber using the coating device, and in particular, to a coating device suitable for applying a resin on the bare optical fiber at a high drawing speed (for example, 2000 m/min or more) and a bare optical fiber coating method using the same.
Description of the Related Art
As is well known, in the manufacture of the optical fiber, resin is applied as a protective coating layer on the surface of the bare optical fiber drawn from the optical fiber preform. In this case, there is a case of applying one-layer resin on the surface of the bare optical fiber and a case of applying two-layer resin on the surface of the bare optical fiber. In recent years, two-layer coating has been adopted in many cases. In the two-layer coating, generally, a first layer (primary layer) using a material with a low Young's modulus is provided on the surface of the bare optical fiber, and a second layer (secondary layer) using a material with a high Young's modulus is provided on the outer side of the first layer.
As a method of applying the two-layer coating, there is a wet on dry coating in which the resin of the primary layer is applied and cured and then the resin of the secondary layer is applied and cured and a wet on wet coating in which the resin of the primary layer and the resin of the secondary layer are collectively applied and cured. Therefore, the independent coating method will be described first herein.
As an apparatus for manufacturing the optical fiber by performing the two-layer coating using the independent coating method, an apparatus shown in FIG. 23 has been widely used. An optical fiber manufacturing apparatus 10 is configured to include a drawing furnace 14, a cooling device 18, a first resin coating device (primary coating device) 20A, a first curing device 22A, a second resin coating device (secondary coating device) 20B, a second curing device 22B, a take-up device 26, and a winding device 27. The drawing furnace 14 heats and melts a preform (optical fiber preform) 12 formed of silica based glass or the like. The cooling device 18 cools a bare optical fiber 16 that is linearly pulled out (drawn) downward from the drawing furnace 14. The first resin coating device 20A coats the cooled bare optical fiber 16 with a resin for first protective coating (primary coating). The first curing device 22A is provided, as necessary, in order to cure the primary coating resin that has been applied by the first resin coating device 20A. The second resin coating device 20B performs coating with a resin for second protective coating (secondary coating). The second curing device 22B is provided, as necessary, in order to cure the secondary coating resin that has been applied by the second resin coating device 20B. The take-up device 26 takes up an optical fiber 24 having the secondary coating resin that has been cured. The winding device 27 winds the optical fiber for which coating has been completed.
When manufacturing the optical fiber using such an optical fiber manufacturing apparatus, the optical fiber preform 12 that is a source of the bare optical fiber 16 is inserted into the drawing furnace 14, and the optical fiber preform 12 is softened and melted by being heated to the temperature of 2000° C. or higher in the drawing furnace 14. Then, the bare optical fiber 16 is pulled out (drawn) downward from the bottom of the drawing furnace 14 while extending the optical fiber preform in a high-temperature state, and the bare optical fiber 16 is cooled to a temperature, at which resin coating is possible, by the cooling device 18. The bare optical fiber 16 has a wire diameter of 125 μm, for example. Then, the bare optical fiber 16 cooled to the required temperature is coated with a primary coating resin by the first resin coating device 20A, and the primary layer is cured by the first curing device 22A. Then, a secondary coating resin is applied on the outer side of the primary layer by the second resin coating device 20B, and the secondary layer is cured by the second curing device. Then, the optical fiber 24 that has been subjected to the two-layer coating is wound on the winding device 27 through the take-up device 26. As the primary coating resin and the secondary coating resin, urethane acrylate based ultraviolet curable resin, silicon based thermoplastic resin, and the like are used. The viscosity of each resin in a liquid state is approximately 0.1 Pa·s to 5 Pa·s at the temperature during the coating.
On the other hand, as an apparatus for manufacturing the optical fiber by performing two-layer coating using the collective coating method, an apparatus shown in FIG. 24 has been widely used.
In this case, one resin coating device 20C capable of performing simultaneous coating of the resin for first protective coating (primary coating) and the resin for second protective coating (secondary coating) is provided for the resin coating. In addition, in order to cure the primary coating resin and the secondary coating resin that have been applied by the resin coating device 20C, one curing device 22C is provided as necessary. The configuration other than these is the same as that of the apparatus shown in FIG. 23, and the same reference numerals are given to the same components as the components shown in FIG. 23.
When manufacturing the optical fiber using the optical fiber manufacturing apparatus shown in FIG. 24, the optical fiber preform 12 is softened and melted by being heated to the temperature of 2000° C. or higher in the drawing furnace 14, in the same manner as described in FIG. 23. Then, the bare optical fiber 16 pulled out from the bottom of the drawing furnace 14 is cooled by the cooling device 18. Then, the bare optical fiber 16 cooled to the required temperature is coated with the primary coating resin and the secondary coating resin by the resin coating device 20C, and the primary layer and the secondary layer are simultaneously cured by the curing device 22C. Then, the optical fiber 24 that has been subjected to the two-layer coating is wound on the winding device 27 through the take-up device 26.
Incidentally, in recent years, in the optical fiber manufacturing process, in order to improve productivity, the drawing speed when manufacturing the optical fiber is set to be significantly higher than that in the related art. Due to the increase in the drawing speed, a coating quality problem is likely to occur when manufacturing the optical fiber by coating the bare optical fiber with a resin. For example, a protective coating layer has an uneven thickness (coating thickness in the circumferential direction in the cross-section of the optical fiber becomes uneven), or variations in thickness deviation in the longitudinal direction of the optical fiber occur, or the outer diameter of the optical fiber after coating becomes uneven (coating thickness in the longitudinal direction of the optical fiber becomes uneven). Therefore, for the resin coating process, various improvements have been conventionally attempted (for example, JAPANESE UNEXAMINED PATENT APPLICATION, FIRST PUBLICATION NO. H9-132437, JAPANESE UNEXAMINED PATENT APPLICATION, FIRST PUBLICATION NO. H9-255372, and JAPANESE PATENT (GRANTED) PUBLICATION NO. 3238105).
JAPANESE UNEXAMINED PATENT APPLICATION, FIRST PUBLICATION NO. H9-132437 discloses making improvements in the coating device in order to make the thickness uniform while sufficiently ensuring the thickness of the coating resin. That is, JAPANESE UNEXAMINED PATENT APPLICATION, FIRST PUBLICATION NO. H9-132437 discloses that, in order to obtain the improved coating quality of the optical fiber even if resin coating is performed at a high drawing speed in a state in which there is slight eccentricity or a slight inclination in a nozzle (die), a plurality (for example, two) of nozzles (dies) are used in the coating device, these nozzles have the same axis, and any (one in the case of two nozzles) of these nozzles has a tapered shape. In addition, for the hole diameters of these nozzles, an optimal relationship therebetween has been derived. In addition, it is disclosed that, by making the coating thickness based on the coating through a die having a tapered portion as small as possible and holding the first and second nozzles coaxially, disturbances due to abnormalities, such as the eccentricity of the tapered portion, are reduced and accordingly the coating resin is applied with a uniform thickness overall.
JAPANESE UNEXAMINED PATENT APPLICATION, FIRST PUBLICATION NO. H9-255372 discloses a coating device for optical fibers that can apply a resin while preventing the occurrence of thickness deviation even if there is slight unevenness in the flow of resin toward the optical fiber in the coating die. That is, in JAPANESE UNEXAMINED PATENT APPLICATION, FIRST PUBLICATION NO. H9-255372, a fiber alignment die is provided between a nipple and the next coating die, a resin supply passage for an alignment die is formed between the nipple and the fiber alignment die, and resin is supplied from the resin supply passage for an alignment die into the die hole of the next coating die through the die hole of the fiber alignment die. Through such a configuration, resin is supplied to the center of the die hole of the coating die from the die hole of the fiber alignment die immediately above the die hole of the coating die. Therefore, it is disclosed that the flow of resin in the circumferential direction of the optical fiber is made to be almost uniform in the die hole of the fiber alignment die. By adopting such a structure, the flow of resin in a merging portion (meniscus) between the optical fiber and the resin and the circulation flow of resin in the tapered portion of the die hole of the coating die can be separated from each other by the fiber alignment die. Accordingly, it is possible to prevent the interference between the upper and lower resin flows. Therefore, it is disclosed that, even if there is slight unevenness in the flow of resin in the die hole of the coating die, the unevenness is canceled, and accordingly, it is possible to apply the resin while preventing the occurrence of thickness deviation.
Additionally, JAPANESE PATENT (GRANTED) PUBLICATION NO. 3238105 discloses making improvements in a method of coating the optical fiber with a resin and a resin coating device for an optical fiber so that it is possible to apply the resin uniformly at high speed. That is, in JAPANESE PATENT (GRANTED) PUBLICATION NO. 3238105, there are provided at least a terminal side coating die having a die hole formed by a tapered hole portion and a land portion, a nipple having a nipple hole, and an intermediate side coating die provided between the coating die and the nipple and having a die hole formed by only a land portion. The terminal side coating die, the intermediate side coating die, and the nipple are provided so as to coaxially overlap each other. The hole diameter of the land portion of the intermediate side coating die is set to be smaller than the inlet diameter of the tapered hole portion. In the combined body thereof, an annular resin reservoir chamber is provided concentrically for the nipple hole. In this state, a resin is applied on the optical fiber while a portion of the resin on the inlet side of the tapered hole portion of the terminal side coating die is returned to the resin reservoir chamber through a resin path provided between the intermediate side coating die and the terminal side coating die.
In this manner, at the time of coating at a high drawing speed, it is possible to suppress the disturbance of the circulation flow in the coating die and to stabilize the meniscus formed in the nipple hole exit. As a result, it is disclosed that it is possible to apply the resin uniformly even at high speed.
The above techniques that have been conventionally proposed have the following points in common.
The intermediate die is provided between the nipple and the coating die.
The hole shapes and hole diameters of the coating die and the intermediate die are optimized.
Resin flows in the resin path between the nipple and the intermediate die (hereinafter, referred to as a “first resin path”) and the resin path between the intermediate die and the coating die (hereinafter, referred to as a “second resin path”) are improved to stabilize the resin flow, in particular, the circulation flow, in each place.
As described above, in order to prevent the occurrence of problems, such as thickness deviation of the resin coating layer, even at the high drawing speed, improvements in the resin coating device (coating device) have also been made in the conventional techniques disclosed in PTLs 1 to 3. In practice, however, it has been difficult to prevent the occurrence of the problems, such as thickness deviation, at the high drawing speed using the conventional techniques.
That is, in the techniques disclosed in PTLs 1 and 2, it has been difficult in practice to control the resin flow in the first and second resin paths in many cases. In the technique disclosed in JAPANESE PATENT (GRANTED) PUBLICATION NO. 3238105, resin should be returned to the resin reservoir in the second resin path. However, there is a high possibility that the resin will not be returned to the resin reservoir uniformly due to a relationship, such as a pressure difference. In addition, a resin flow difference (difference in the flow rate or the amount of flow) in the circumferential direction in a merging portion of the resin flows in the first and second resin paths and the circulation flows generated in the intermediate die hole and the coating die hole may disturb the circulation flow in the die hole. Due to these influences, especially at the time of high drawing speed, the circulation flow of the resin in the coating die hole that is the most important consideration at the time of resin coating is disturbed and the resin flow becomes unstable and uneven, and accordingly, the resin coating thickness becomes uneven. As a result, problems, such as thickness deviation, variations in thickness deviation in the longitudinal direction of the optical fiber, or non-uniform coating diameter, were likely to occur.
The present invention has been made in view of the situation described above, and it is an object of the present invention to provide a resin coating device and a resin coating method that do not cause the thickness deviation of a coating layer, variations in thickness deviation in the longitudinal direction of the optical fiber, or variations in the fiber diameter even when the bare optical fiber is inserted into the coating device at high speed, that is, even when the bare optical fiber is coated at a high drawing speed.